Purpose:
The ocular tunic is comprised of the cornea and sclera which cooperatively maintain refractive homeostasis under fluctuating intraocular pressure (IOP) and prevent excessive deformation of the retina and optic nerve head. Corneo-scleral material characteristics are governed largely by the load-bearing connective tissues, mainly type I collagen fibrils. The purpose of this work was to obtain specimen-specific quantitative measurements of collagen fibril architecture across whole human ocular tunics, in order to provide a microstructural framework for future modelling of biomechanical behaviour in intact eye globes.

Methods:
Wide-angle x-ray scattering (WAXS) was used to quantify collagen fibril architecture at a resolution of 0.5mm across flattened human ocular tunics. Three structural parameters were measured at each sampled point: (i) the preferred alignment direction and associated angular distribution of collagen fibrils, (ii) the relative total collagen content, and (iii) the relative proportion of preferentially aligned collagen (anisotropy). The two-dimensional data-sets were re-located onto the globe surface using the specimen-specific 3D geometry of the intact eye globe gathered under whole eye inflation with digital image correlation prior to x-ray analysis.

Results:
The observed collagen fibril anisotropy across the ocular tunic reflects corneo-scleral stress patterns predicted in response to the action of the IOP and extra-ocular muscles. The main identified structural features relevant to ocular behaviour were: (i) circumferential collagen at the limbus and circumscribing the optic nerve head and (ii) uniaxial collagen aligned along the rectus and oblique muscle insertions.

Conclusions:
Accurate, specimen-specific quantification of collagen architecture in the ocular tunic is an important step towards building microstructure-based biomechanical models of the intact human eye globe. Such models will provide future insight into the progression of ocular diseases affecting the cornea-scleral connective tissues, including glaucoma and myopia, as well as the eye’s biomechanical response to surgical intervention.